1. Field of the Invention
This invention relates to slotted containers (RSC""s) and bulk boxes. More specifically, the invention improves the performance of slotted containers and bulk boxes susceptible to cyclic changes in ambient humidity and temperature with a repulpable water vapor barrier in the outer linerboard.
2. Prior Art
Large corrugated cardboard boxes are commonly used in the storage and shipment of bulk quantities of materials. These bulk boxes are typically designed to hold up to 2500 pounds, or more, of product, and may be stacked three or more high. During transportation and storage they may be subjected to rough handling and adverse ambient conditions. For example, during storage they are usually placed in a warehouse, which does not have a controlled atmosphere, whereby the boxes are subjected to cyclic changes in temperature and humidity.
Under conditions of high ambient moisture, and particularly cyclic changes in ambient moisture and temperature, the pulp fibers in the corrugated material experience the effect of hysteresis, whereby the fibers swell and contract as the level of moisture changes. Each cycle weakens the fibers. As the fibers weaken, the side walls of the box bulge outwardly, interfering with storage and handling, and in some cases resulting in failure of the box.
The serious loss of strength and stiffness resulting from these cyclic conditions, and the consequent dimensional change and potential failure of the box, shortens the shelf life and limits the number of boxes, which can be stacked.
Historically, fiber loading has been the only means to combat the deleterious effect of hysteresis. The total weight a box will have to support during its life cycle is calculated and then multiplied by a safety factor. The amount of the safety factor is based on the conditions the box is expected to endure. In some instances, boxes are produced with a safety factor of 8:1, i.e., the box is produced eight times as strong as needed. This obviously increases the amount of material and thus the cost of construction of the box.
Efforts have been made in the prior art to improve the performance of corrugated bulk boxes, as exemplified by the following patents.
U.S. Pat. No. 1,161,818 discloses a corrugated shipping container having an outer liner formed of plural laminated plies to improve the resistance of the container to ambient conditions and rough handling. The outer liner is described as being a plural ply fiber paste-board that is preferably of a solid, firm and hard texture comprising several plies of paper united together by silicate of soda or other strong paste or cement. Page 1, lines 100-110, and page 2, lines 1-10.
U.S. Pat. No. 3,616,010 discloses a corrugated shipping container having a thermoplastic film to make it moisture resistant for use in high humidity conditions. More specifically, a roll 16 of kraft paper bag stock in the range of 17 lbs/MSF coated with a layer 22 of thermoplastic material such as polyethylene, polypropylene, etc., is laminated with a kraft linerboard 26 of, e.g., 26 lbs/MSF, to form a laminated linerboard 34, which is then laminated with the corrugated medium 42. The PE imparts moisture resistance to the structure, but the moisture resistant material, PE, is not recyclable.
U.S. Pat. No. 5,562,980 teaches a multi-layer repulpable paper or paperboard construction that has good moisture vapor barrier qualities and mechanical strength, and that has utility as a wrapper for paper rolls or as a linerboard for corrugated boxes. The barrier material comprises a multi-layer laminant interposed between two paper sheets forming linerboard. This patent uses polyvinylidene chloride (PVdC) as the moisture barrier material. The patentee recognizes that with this material it is difficult to achieve a continuous film that provides the desired moisture protection. Moreover, the patentee recognized that because PVdC is partially soluble in water, prior to his invention it had to be applied at high rates in order to achieve an acceptable level of moisture protection. Further, PVdC has a negative impact on the environment and can cause problems when discharged from a recycling facility into a waste effluent stream. The patentee discovered that by the use of appropriate primer and laminate materials, the moisture barrier material, PVdC, can be applied at a low rate, enhancing recyclability without impairing its qualities as a barrier layer. Clay is disclosed as a suitable inorganic pigment useful in the primer layer.
U.S. Pat. No. 5,609,293 discloses a corrugated container for prolonging the life of fresh fruits and vegetables by, at least in part, controlling the gas permeability of the container, and to this end incorporates a plastic-paperboard liner construction having a kraft paper outer layer 5, an intermediate polymeric film layer 6, and an inner kraft layer 4, laminated to a corrugated medium 8. It appears that this liner construction is placed on the inside of the container. The examples identify polyethylene as the polymeric material used in the liner.
U.S. Pat. No. 5,698,295 discloses a corrugated box with inner and outer liners coated with a repulpable moisture resistant material to improve the performance of the container under high humidity conditions. The moisture resistant property of the container is imparted by coatings 20 and 22, respectively, of repulpable moisture resistant material on the outer faces of the liners, applied by rod coating. The outer coating accepts printing. An example of a suitable coating 22 for the outer surface is identified as a white, water-based, tri-polymer blend of the type sold by Progressive, Inc. of Shreveport, La., under the designation W-108 White Coating. This coating is applied to 69 # linerboard, with a corrugated medium of 33 # weight.
The prior art which teaches the use of a moisture barrier material on a bulk box either does not use a readily repulpable barrier material, and/or uses a material which can have a negative environmental impact, and/or uses materials and processes which are unacceptably expensive.
Accordingly, there is need for a bulk box having an inexpensive and readily repulpable water vapor barrier material incorporated therein to increase resistance of the corrugated material to the effects of ambient humidity, and especially to cyclic changes in ambient moisture and temperature, thereby improving the performance of the box under these conditions.
As a box fatigues under top load, its sidewalls begin to bulge outward. The degree of bulge is directly related to the top to bottom compression the box can withstand. The more resistant to bulge a box is, the more weight it can support. The measurement of bulge over time will relate to the condition of a box under load as well as the potential life cycle of that box.
The present invention comprises a bulk box or slotted container having an inexpensive and readily repulpable water vapor barrier material incorporated therein to increase resistance of the corrugated material to the effects of ambient humidity, and especially to cyclic changes in ambient moisture and temperature. The improved performance under these conditions will allow for a reduction in the safety factor in any slotted container or bulk box enabling less pulp fiber to be used in the construction of the box.
The bulk box or slotted container of the invention has enhanced stacking strength, especially in cyclic environments, and by reducing or eliminating the transmission of moisture through the corrugated material, a lighter grade of box can be produced which performs equally as well as or better than the heavier weight boxes previously required.
Applicant previously developed a repulpable roll wrap to replace the polyethylene coated and laminated roll wrap used by fine paper mills to wrap large rolls of paper. This roll wrap uses a repulpable Michelman MVTR barrier as a bonding agent in a two ply linerboard laminate.
Applicant has discovered that the repulpable Michelman MVTR barrier used in its roll wrap can be incorporated in the outer linerboard on a bulk box to impart moisture resistance to the bulk box and improve its performance while requiring less pulp fiber than prior constructions not employing the water vapor barrier.
More specifically, the outer liner of a bulk box according to the present invention comprises two plies of either 26 #/MSF or 33 #/MSF kraft paper (linerboard) bonded together using the xe2x80x9cVaporcoatxe2x80x9d 117B water vapor barrier material produced by Michelman, Inc. of Cincinnati, Ohio.
In a preferred construction, the water vapor barrier is applied with a rod coater to the felt side of the inner ply of the linerboard laminate, and the coated inner ply is mated to the wire side of the outer ply. This leaves the felt side of the outer ply available for printing. This laminated structure also has significantly higher ring crush strength than a non-laminated comparable weight linerboard.
The Michelman xe2x80x9cVaporcoatxe2x80x9d barrier is a composition of polymers and ground up mica which forms the moisture vapor resistant layer. In a specific construction, the barrier coating is applied at the rate of 10-12 wet pounds per one thousand square feet (10-12 wet #/MSF).
It is possible that other readily repulpable water vapor barrier materials may perform well in the invention, and applicant should not be limited to the specific Michelman xe2x80x9cVaporcoatxe2x80x9d 117B material. Additionally, a laminated construction of the outer liner may not be required for all applications, and a single ply linerboard coated with the moisture barrier may perform satisfactorily in some applications.
Bulk boxes produced in accordance with the invention show a 700% improvement in their resistance to cyclic creep as compared with a standard construction under identical conditions. The invention thus results in significantly improved performance and reduction in the cost of bulk boxes and certain RSC""s.